Device for electric field control

ABSTRACT

A device for controlling an electric field at a high voltage component including a resistive layer for field control, an insulating layer arranged on the resistive layer and a semi-conducting or conducting layer arranged on the insulating layer. The three layers meet at a triple point where the insulating layer ends. An interface between the resistive layer and the insulating layer makes in the triple point an angle to the semi-conducting or conducting layer of 60°-120°.

TECHNICAL FIELD OF THE INVENTION AND PRIOR ART

The present invention relates to a device for controlling an electricfield at a high voltage component, said device comprising a resistivelayer adapted, for field controlling purposes, to be arranged along asaid component and which at one end is adapted to be electricallyconnected to a live high voltage part of the component and, at its otherend, adapted to be electrically connected to ground potential, aninsulating layer arranged on said resistive layer and extending outsidethereof from said one end towards said other end while ending by atapering field controlling geometry without reaching at a distance tosaid other end, as well as a semi-conducting or conducting layerarranged on the insulating layer and extending outside thereof past theend of the insulating layer towards said other end of the resistivelayer, so that said resistive layer, said insulating layer and saidsemi-conducting or conducting layer meet at a triple point at said endof the insulating layer.

The high voltage component may carry an alternating voltage or a directvoltage, and it may for instance be a high voltage cable, in which thedevice may be used in a cable joint or a cable termination. Other highvoltage components are also conceivable, such as bushings in switchgearsand vacuum breakers.

A device of this type is known through for instance WO 00/74191 A1. Adevice of this type is used for distributing the electric field wherethis is crucial and by that avoiding electric field concentrations anddamaging of different type of equipments, such as cables. The electricpotential between the live part and ground is distributed by means of amaterial with a suitable resistance forming said resistive layer. Bycombining the resistive field control and a geometrical field controlobtained through said insulating layer the risk of harmful chargesbuilding up and of high stresses upon said component, such as a cable,arising at rapid changes of the voltage thereof, is reduced with respectto such a device having only a resistive field control through a saidresistive layer.

However, when using known electric field control devices of the typedefined in the introduction for voltages above a determined limit, whichis depending on the material used for the different layers, the devicewill be damaged limiting the use of such a device with respect to thelevel of said high voltage. It has turned out that the device will bedamaged in the region of said triple point, where the insulating layer,the semi-conducting or conducting layer and the resistive layer meet.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an electric fieldcontrol device of the type defined in the introduction which may be usedfor higher voltages than such electric field control devices alreadyknown without being damaged.

This object is according to the invention obtained by providing such adevice in which said resistive layer and said insulating layer are closeto said end of the insulating layer designed so that the interfacebetween these two layers ends in said triple point by making an angle tosaid semi-conducting or conducting layer of 60°-120°.

It has been found that said angle of said interface strongly influencesthe behaviour of the materials in said triple point, and thatconsiderably higher voltages may be taken by these materials withoutdamaging the device in this region by selecting said angle within saidinterval thanks to the advantageous extension of the equipotential linesin the resistive layer and the insulating layer close to said triplepoint. This also means that for a given voltage level less material maybe used for said resistive layer while still being able to take thevoltage, which results in a considerable reduction of costs, since thisfield controlling material is expensive. This angle is preferablyobtained by the new approach to work and shape the resistive fieldcontrolling layer at said triple point.

According to an embodiment of the invention the resistive layer and saidinsulating layer are close to said end of the insulating layer designedso that said interface therebetween ends in said triple point by makinga said angle to said semi-conducting or conducting layer of 70°-110°,advantageously 80°-100°, and preferably 85°-95°. It has been found thatit is in most cases with respect to the stresses on said triple pointmost favourable to have said angle close to 90°, but angles within saidintervals may be chosen for ease of manufacturing or other reasons whilestill giving the desired result with respect to preventing damage in theregion of the triple point.

According to another embodiment of the invention said insulating layerends in the extension thereof from said one end towards said other endby a substantially sharp edge forming a transversal surface continuinginto the resistive layer for forming a step in said resistive layer andmaking this thinner in the direction towards said other end forobtaining said angle in said triple point. A device of this type is easyto manufacture and results in comparatively small waste of the expensivefield controlling material of said resistive layer.

According to another embodiment of the invention said resistive layerhas a recess in the region of said triple point receiving an end portionof said insulating layer for obtaining said angle in said triple point.

According to another embodiment of the invention said resistive layerhas a tip-like elevation in the region of said triple point with the tipending in said triple point and having said insulating layer on one sidethereof and said semi-conducting or conducting layer on the other sidethereof for obtaining said angle in said triple point. Such a tip-likeelevation may be manufactured while very reliably defining said angle tothe value desired, but the manufacturing thereof will result in agreater waste of said field controlling material than for the twoprevious embodiments.

According to another embodiment of the invention said superimposedresistive layer, insulating layer and semi-conducting or conductinglayer form a sleeve for receiving said high voltage component with theresistive layer being the innermost layer of the sleeve.

According to another embodiment of the invention the device is adaptedto control electric fields at high voltage components having analternating voltage or a direct voltage on said live high voltage part,and it is adapted to control electric fields for high voltages beingabove 1 kV, above 50 kV, above 100 kV, 130 kV-400 kV, 200 kV-400 kV or250 kV-350 kV. The invention is the more interesting the higher thevoltage is, although it may also be favourable for voltages being low inthis context, which means for instance in the order of 10 kV. Theinvention is especially applicable to voltages in the region of 100kV-400 kV, for which damage of the region of said triple pointconstitutes an obstacle to raising the voltage further in for instancehigh voltage cable joints or cable terminations.

The invention also relates to a cable joint for joining two high voltageDC-cables or AC-cables provided with a device for electric field controlaccording to the invention, as well as a cable termination forterminating a high voltage DC-cable or AC-cable provided with a devicefor electric field control according to the invention. These are twopreferred uses of a device for controlling an electric field of thistype, since they are often sensitive parts of a large power transmissionsystem, in which there is an ongoing attempt to raise the voltages forreducing power losses.

The invention also relates to a method for manufacturing a device forcontrolling an electric field comprising the following steps: theresistive layer is wound around a carrier in a number of sub-layers andthen ground to the desired shape, the insulating layer is then woundoutside the resistive layer in a number of sub-layers and then ground tothe desired shape, and the semi-conducting or conducting layer is woundin sub-layers outside the insulating layer and the resistive layer, andthe resistive layer and/or the insulating layer are in the previoussteps ground for obtaining said angle in said triple point.

Further advantages as well as advantageous features of the inventionwill appear from the following description.

BRIEF DESCRIPTION OF THE DRAWINGS

With reference to the appended drawings, below follow a specificdescription of embodiments of the invention cited as examples.

In the drawings:

FIG. 1 is a cross-section view of a cable joint provided with anelectric field control device according to an embodiment of the presentinvention,

FIG. 2-4 are enlarged simplified cross-section views of a part ofelectric field control devices according to different embodiments of theinvention in the region of said triple point,

FIGS. 5 and 6 are views of the embodiments according to FIGS. 2 and 4illustrating the extension of the equipotential lines in the case of ahigh voltage on a cable connected to the cable joint, and

FIG. 7 is a simplified view illustrating a cable termination providedwith an electric field control device according to an embodiment of theinvention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

A cable joint 1 is shown in cross-section in FIG. 1. This cable jointhas an inner resistive layer 2 with electric field controllingproperties adapted to be arranged along a cable 3 and to be at one end 4connected to a live high voltage part of the cable and at it's other end5 electrically connected to ground potential by being connected to theouter semi-conducting sheath of the cable.

An insulating layer 6 is arranged outside the resistive layer and extendtowards said other end 5 while ending by a tapering field controllinggeometry 7 without reaching and at a distance to this end 5.

A semi-conducting or conducting layer 8 is arranged outside theinsulating layer and extend past the end of the insulating layer towardssaid other end 5 of the resistive layer, so that the resistive layer,the insulating layer and the semi-conducting or the conducting layermeet at a triple point 9 at said end 10 of the insulating layer. Thematerial of the different layers 2, 6 and 8 may for example beEPDM-rubber or silicone rubber with different additives giving thedifferent layers the properties desired.

A deflector 11 having nothing to do with the present invention isarranged in the region of the end of a live part of the respective cablefor protection against discharges and the like.

The appearance of the resistive layer 2, the insulating layer 6 and thesemi-conducting or conducting layer 8 in the region of said triple point9 is shown by an enlargement A. This corresponds to the embodiment shownin FIG. 2, which will now be described. The live part or conductor 12and the insulating layer or sheath 13 as well as the outersemi-conducting sheath 14 of the cable have been shown here, and in thisfigure and FIGS. 3 and 4 only a half of one end of the cable joint isshown in cross-section. It is shown how the insulating layer 6 ends by asubstantially sharp edge forming a transversal surface 15 continuinginto the resistive layer while making this thinner in the directiontowards said other end 5. This result in an angle made by the interfacebetween the resistive layer 2 and the insulating layer 6 to saidsemi-conducting or conducting layer 8 in said triple point of close to90° resulting in the advantages described above. This geometry may beeasily obtained by grinding the resistive layer and the insulating layerafter having been wound as described above.

FIG. 3 is a view similar to FIG. 2 of an embodiment differing from theembodiment according to FIG. 2 by the fact that here a recess 20 hasbeen created in said resistive layer close to the triple point 9 andthis recess receives an end portion 21 of the insulating layer 6 forobtaining a said angle close to 90°.

FIG. 4 shows how said angle in said triple point is instead obtained byproviding the resistive layer with a tip-like elevation 17 with a tipdefining said triple point with the insulating layer and thesemi-conducting or conducting layer arranged on each side of said tip.

FIGS. 5 and 6 illustrates how the equipotential lines 18 will extendvery smoothly in the region of said triple point 9 thanks to theappearance of this triple point with the angle between said interface 16and said semi-conducting or conducting layer 8 close to 90°, such as70-110°. This means that for a given material and thickness of theresistive layer the voltages of said live part of the cable may beincreased considerably with respect to cable joints according to theprior art without causing damage of the cable joint in the region of thetriple point 9.

FIG. 7 illustrates a cable termination 19 having an electric fieldcontrol device according to the present invention, and parts of thisdevice corresponding to such of the device according to FIG. 1 have beenprovided with the same reference numerals with addition of a prime. Saidtriple point is also crucial for the strength of such a cabletermination, and the design of the different layers in connection withthis triple point results in the same advantages as described above fora cable joint.

The invention is of course not in any way restricted to the embodimentsdescribed above, but many possibilities to modifications thereof wouldbe apparent to a person with ordinary skill in the art without departingfrom the basic idea of the invention as defined in the appended claims.

1-10. (canceled)
 11. A device for controlling an electric field at ahigh voltage component, said device comprising: a resistive layerarranged along said component and which at a first end is configured tobe electrically connected to a live high voltage part of the componentand, at a second end, configured to be electrically connected to groundpotential, an insulating layer arranged on said resistive layer andextending outside thereof from said first end towards said second end,the insulating layer terminated by a tapering field controlling geometrywithout reaching the second end and at a distance to said second end,and a semi-conducting or conducting layer arranged on the insulatinglayer and extending outside thereof past an end of the insulating layertowards said second end of the resistive layer, so that said resistivelayer, said insulating layer and said semi-conducting or conductinglayer meet at a triple point at said end of the insulating layer,wherein in a region close to said end of the insulating layer saidresistive layer and said insulating layer are designed so that aninterface between the resistive layer and the insulating layer at saidtriple point makes an angle to said semi-conducting or conducting layerof 60°-120°.
 12. The device according to claim 11, wherein saidresistive layer and said insulating layer are close to said end of theinsulating layer designed so that said interface therebetween ends insaid triple point by making a said angle to said semi-conducting orconducting layer of 70°-110°.
 13. The device according to claim 11,wherein said insulating layer ends in an extension thereof from saidfirst end towards said second end by a substantially sharp edge forminga transversal surface continuing into the resistive layer for forming astep in said resistive layer and making the resistive layer thinner in adirection towards said second end for obtaining said angle in saidtriple point.
 14. The device according to claim 11, wherein saidresistive layer comprises a recess in a region of said triple pointreceiving an end portion of said insulating layer for obtaining saidangle in said triple point.
 15. The device according to claim 11,wherein said resistive layer comprises a tip-like elevation in a regionof said triple point with the tip-like elevation ending in said triplepoint and having said insulating layer on one side thereof and saidsemi-conducting or conducting layer on the other side thereof forobtaining said angle in said triple point.
 16. The device according toclaim 11, wherein said superimposed resistive layer, said insulatinglayer and said semi-conducting or conducting layer form a sleeve forreceiving said high voltage component with the resistive layer being aninnermost layer of the sleeve.
 17. The device according to claim 11,wherein the device is adapted to control electric fields at high voltagecomponents having an alternating voltage or a direct voltage on saidlive high voltage part, and wherein the device is adapted to controlelectric fields for high voltages being above 1 kV, above 50 kV, above100 kV, 130 kV-400 kV, 200 kV-400 kV or 250 kV-350 kV.
 18. The deviceaccording to claim 11, wherein said resistive layer and said insulatinglayer are close to said end of the insulating layer designed so thatsaid interface therebetween ends in said triple point by making a saidangle to said semi-conducting or conducting layer of 80°-100°.
 19. Thedevice according to claim 11, wherein said resistive layer and saidinsulating layer are close to said end of the insulating layer designedso that said interface therebetween ends in said triple point by makinga said angle to said semi-conducting or conducting layer of 85°-95°. 20.A cable joint for joining two high voltage DC-cables or AC-cables, thecable joint comprising: a device for electric field control at each endthereof, each device for electric field control comprising a resistivelayer arranged along a component and which at a first end is configuredto be electrically connected to a live high voltage part of thecomponent and, at a second end, configured to be electrically connectedto ground potential, an insulating layer arranged on said resistivelayer and extending outside thereof from said first end towards saidsecond end, the insulating layer terminated by a tapering fieldcontrolling geometry without reaching the second end and at a distanceto said second end, and a semi-conducting or conducting layer arrangedon the insulating layer and extending outside thereof past an end of theinsulating layer towards said second end of the resistive layer, so thatsaid resistive layer, said insulating layer and said semi-conducting orconducting layer meet at a triple point at said end of the insulatinglayer, wherein in a region close to said end of the insulating layersaid resistive layer and said insulating layer are designed so that aninterface between the resistive layer and the insulating layer at saidtriple point makes an angle to said semi-conducting or conducting layerof 60°-120°.
 21. A cable termination for terminating a high voltageDC-cable or AC-cable, the cable termination comprising: a device forelectric field control comprising a resistive layer arranged along acomponent and which at a first end is configured to be electricallyconnected to a live high voltage part of the component and, at a secondend, configured to be electrically connected to ground potential, aninsulating layer arranged on said resistive layer and extending outsidethereof from said first end towards said second end, the insulatinglayer terminated by a tapering field controlling geometry withoutreaching the second end and at a distance to said second end, and asemi-conducting or conducting layer arranged on the insulating layer andextending outside thereof past an end of the insulating layer towardssaid second end of the resistive layer, so that said resistive layer,said insulating layer and said semi-conducting or conducting layer meetat a triple point at said end of the insulating layer, wherein in aregion close to said end of the insulating layer said resistive layerand said insulating layer are designed so that an interface between theresistive layer and the insulating layer at said triple point makes anangle to said semi-conducting or conducting layer of 60°-120°.
 22. Amethod for producing a device for controlling an electric field, themethod comprising: winding a resistive layer around a carrier in anumber of sub-layers and then grinding the resistive layer to a desiredshape, winding an insulating layer outside the resistive layer in anumber of sub-layers and then grinding the insulating layer to a desiredshape, and winding a semi-conducting or conducting layer in sub-layersoutside the insulating layer and the resistive layer, wherein at leastone of the resistive layer or the insulating layer are ground forobtaining an angle in a triple point.